Ultrahigh strength steels

ABSTRACT

AN ULTRAHIGH STRENGTH MATERIAL MADE FROM ORDINARY 188 STAINLESS STEEL HAS A TENSIL STRENGTH IN EXCESS OF 400,000 P.S.I. THE METHOD OF PRODUCING THE ULTRAHIGH STRENGTH IS ACCOMPLISHED BY THERMO-MECHANICAL OPERATIONS. THE MATERIAL CAN HAVE ANY DESIRED GEOMETRIC CROSS SECTION CONFIGURATION AND IS ADAPTABLE FOR USE AS A SPRING MATERIAL.

oct. 17, 1972 l -f J. NuNEs :ET AL ULTRAHIGH STRENGTH STEELS Filed Sept,21,

d H 4 u k I 4 Kl x K *r r1 d d H P E T S P E T S CYCLE OF OPERATIONS ORSTEPS mveNToRs M-v ATTORNEY United States Patent Office Patented Oct.17, 1972 3,698,963 ULTRAHIGH STRENGTH STEELS John Nunes, Lexington, andAlbert D. Martin, Groton, Mass., assignors to Brunswick CorporationFiled Sept. 21, 1970, Ser. No. 73,962 Int. Cl. C21c 39/00; C21d 7/02,9/02 U.S. Cl. 14S-12.4 14 Claims ABSTRACT OF THE DISCLOSURE An ultrahighstrength material made from ordinary 18- 8 stainless steel has a tensilestrength in excess of 400,000 p.s.i. The method of producing theultrahigh strength is accomplished by thermo-mechanical operations. Thematerial can have any desired geometric cross section contiguration andis adaptable for use as a spring material.

BACKGROUND OF THE INVENTION Field of the invention This invention is inthe iield of high strength steels and, more particularly, in the fieldof ultrahigh strength 18-8 stainless steel materials.

Description of the prior art With the invention of adding approximately18% chromium and 8% nickel to a relatively carbon-free iron, rustlesssteel or stainless steel was born. Since that early development in the1910s, many modifications and permutations have been made of the basic18-8 stainless steel. Later, this basic material was classified as anaustenitic stainless steel because other classes of stainless steelsthat were not austenitic were coming into existence. Even more recently,all the stainless steels have been reclassied with this early group of18-8 stainless steels now referred to as AISI type 300 series stainlesssteels. The basic 18-8 stainless steel is now generally referred to asAISI type 302 stainless steel.

Type 302 stainless exhibits an austenitic structure and cannot bereadily or appreciably transformed by heat treatment only into asubstantially martensitic structure. However, many attempts have beentried to make type 302 stainless steel stronger and harder primarily byexotic heat treatments without altering the oxidation and corrosionresistance thereof. In retrospect, it was not expected that suchattempts would work and in essence they have not worked. However, type302 stainless steel and special compositions thereof have beenstrengthened by cold work with achievable results indicating strength upto a maximum of 355,000 p.s.i. for standard type 302 stainless and380,000 p.s.i. for special compositions thereof. These specialcompositions constituting small chemical changes do not significantlyalter the structure of the material.

On the other hand, drastic alterations in chemical compositions havebeen introduced substantially increasing the strength of stainless steelmaterials, yet changing the oxidation and corrosion resistance thereof.In some instances, additional materials were, of necessity, added tothese compositions to restore the oxidation and corrosion resistancecharacteristics. The result of these changes have created new familiesof stainless steels, which are far more expensive and more -limited intheir particular scope of usage than the basic type 302 stainless.

The 355,000 p.s.i. to 380,000 p.s.i. strength levels achievable incurrent type 302 stainless steel alloys are far below the strengthlevels achievable in the highest strength alloy steel materials usablefor springs. Some of these alloy steels are used to make line springs,yet are not conducive for use in oxidizing or other corrosiveatmospheres. Consequently, it has long been recognized that it would beextremely desirable to extend the strength range of type 302 stainlesssteel to provide a good, general high strength stainless steel materialthat could be used for springs, such as in oxidizing or corrosiveatmospheres.

SUMMARY OF THE INVENTION This invention relates to oxidation andcorrosion resistant steels, and is concerned with new and improvedcharacteristics 0f 18-8 stainless steel alloys that provide ultrahighstrength levels from over 400,000 p.s.i. to over 600,000 p.s.i. Thisinvention also relates to a new and improved thermo-mechanical method oftreating such 18-8 stainless steels to achieve these ultrahigh strengthlevels.

Brieliy, an 18-8 stainless steel material is subjected to a series ofdeformation hardening operations always performed below therecrystallization temperature for the material with intermediateanneals. Subsequently, the material is deformation hardened to a veryhigh level of cold work, over The material is further processed by beingsubiected to an intermediate heat treatment to inhibit dynamic recovery,and to obtain an aging response thereby enhancing the strength alreadyachieved from the deformation hardening. The material can be furtherdeformation hardened with intermediate heat treatments to again increasethe strength to higher levels, yet still exhibiting excellent oxidationresistance at temperatures in the 500 degree F. rangeL Thus, suchmaterials will provide excellent spring characteristics and oxidationresistance hitherto unknown in 18-8 stainless steel spring materials.

It is an object of this invention to provide an 18-8 stainless steelwith a strength level in excess of 400,000 p.s.i., yet retaining theoxidation and corrosion resistance that I'is exhibited by this materialin its normal cold-worked state.

It is another object of this invention to provide a method forprocessing 18-8 stainless steel to ultrahigh strength levels bythermo-mechanical treatment of the material.

It is a feature of the invention to provide such ultrahigh strength 18-8stainless steel materials to be used as springs.

It is another feature of this invention to provide an 18- 8 stainlesssteel material having an ultrahigh strength in cross-sectionalconfigurations as desired, e.g., circles, squares, rectangulars,I-shapes, elongated rectilinear shapes, T-shapes, etc.

Still another feature of the invention is to provide an ultrahighstrength 18-8 stainless steel material in an operating range up to about500 degrees F. yet retaining good oxidation resistance.

Yet another feature of the invention is to provide a plurality of 18-8stainless steel filaments, each having a tensile strength in excess of400,000 p.s.i.

Another feature of the invention is to provide a composite materialhaving the plurality of 18-8 stainless steel filaments surrounded bymetal matrix material, said composite exhibiting a tensile strength inexcess of 400,000 p.s.i.

Another feature of the invention is to provide an 18-8 stainless steelWire having an ultra-high strength with a. corrosion resistant coveringsuperior to the core stainless steel.

The above and other and further objects and features of the inventionwill be more readily understood by reference to the following detaileddescription and the accompanying drawing.

DESCRIPTION OF THE DRAWING FIG. l is a cross sectional View of acircular tube surrounding a circular wire;

FIG. 2 is a cross sectional view of a sheathing material having acircular external configuration surrounding a core material having asquare external configuration;

FIG. 3 is a cross sectional view of a sheathing material having acircular external configuration Surrounding a core material having arectangular external configuration;

FIG. 4 is a cross sectional view of a sheathing material having acircular external configuration surrounding a core material having anhexagonal external configuration;

FIG. 5 is a cross sectional view of a sheathing material having acircular external configuration surrounding a core material having anI-shaped external configuration;

FIG. 6 is a cross sectional view of a sheathing material having acircular external coniiguration surrounding a core material having anelongated external configuration;

FIG. 7 is a cross sectional view of the configuration of FIG. 1 whereinthe sheath is drawn down tightly on the core material and having aninterface designated therebetween; and

FIG. 8 is a schematic ilow chart of the process utilized in producingthe high strength stainless steel material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a preferred embodiment ofthis invention, an 18-8 stainless steel core material having across-sectional configuration such as a circle, a square, a rectangle, ahexagon, an I-shaped, an elongated rectilinear shape, etc., is clad orsheathed with a metal such as a nickel-copper alloy, type 310 stainlesssteel nickel base superalloys, cobalt base superalloys, nickel-cobaltalloys, copper base alloys, lead, titanium and its alloys, to form acomposite. The composite is then reduced in cross section so that thesheath tightly adheres to the core. When it is desired that thedistortion of a core shape be held to a minimum, then the interior ofthe sheath should conform to the exterior of the core. The colddeformation may be performed by drawing, swaging, rolling, pressing,squeezing, etc., or any desired combination thereof.

As delined for use in this disclosure, type 302 stainless steel (18-8stainless steel) has the following approximate chemical analysis byweight:

which is substantially the same as the United States Governments AMSSpecification. All constituent elements, except for carbon, that arepresent in less than one percent quantities are considered andcharacterized as minor elements for the purpose of this disclosure. Inaddition, the percent reduction, the percent cold deformed state, thepercent cold worked state, etc., are all the same as the percentreduction in cross sectional area of the material after the last anneal.In other words, a 97.6% cold worked state is the same as a 97.6%reduction in cross sectional area.

It has been found that it is easier to reduce the cross section of thecomposite when the exterior of the sheath is substantially circular,however, other external configurations of the sheath may be used asdesired. The substantially cvircular exterior cross section 8 of such acomposite 7 configuration is shown in FIG. 1 wherein the core 10 issurrounded by a sheath 12 and the exterior surface 11 of the core 10 issubstantially circular and adjacent the interior surface 13 of thesheath 12. In a similar manner, the cross-sectional compositeconfigurations for a square 10A, a rectangle 10B, a hexagon 10C, anI-shape 4 10D, and an elongated rectangle 10E, are shown in FIGS. 2through 6. The sheath 12 is reduced in size tightly on the core toprevent any relative movement between the core 10 and the sheath 12 withan interface 15 provided to promote equal reduction of the core 10 andthe sheath 12, as shown in FIG. 7.

The ratio of the sheath material to the core material depends upon thetypes of materials and configuration. Approximately a 5 to 10% reductionin area is required to provide the initial tight mechanical bond betweenthe, sheath 12 and the core 10. When the core 10 is made of a materialsuch as type 302 stainless steel, and the sheath is made of the materialsuch as a nickel-copper alloy, the composite' is yannealed in `a heattreating furnace at a nate of approximately two seconds -per mil ofdiameter of the composite. The annealing or recrystallizationtemperature must `be suliiciently high to .provide la complete solutionanneal of the' core. For the type 302 stainless and steel nickelcopperalloy composite, 1950 degrees F. -is sufIi-cient to provide the solutionanneal land cause a minor degree of diffusion bonding to occur betweenthe `core 10 and the sheath 12 at the interface 1S further insuring thatthere will be no relative movement between the core and the sheath. Thecomposite -is rapidly quenched as it leaves the annealing furnacepreventing carbide precipitation in the microstructure of the stainlesssteel.

The composite is then subjected to a series of cold reducing steps withintermediate anneals wherein the composites area is reduced at least 75%by cold deformation and preferably by cold deformation after the lastanneal. Each of the intermediate anneals is performed above therecrystallization temperature of the core material. However, thistemperature should be kept as low as possible to prevent extensivediffusion between the core and sheathing materials. It is believed thatthe extreme amount of cold deformation is enhanced because the sheathingsupports the core material, provides a protective coating and functionsas a lubricant. This particular step in the process is extremelyimportant, and heretofore has not been recognized as one of the primarysteps necessary to provide the ultrahigh strength material havingcomplicated geometric cross sectional configurations. It is believedthat for simple geometric shapes, such as an elongated rectangle, asquare, a circle, etc., that the cladded sheath is not required;however, it can be used, if desired.

At this stage in the processing, three different series of operationsmay be employed, depending on the desired Ifinal strength of the corematerial.

In one preferred embodiment, the 75-84% cold deformed composite isheated for approximately four hours (with the permissable time rangingfrom about one half hour to about sixteen hours or more) at atemperature well below the lowest recrystallization or transformationtemperature of the core material; for type 302 stainless material, therange would be about 700 degrees F. to 825 degrees F., and preferably ina narrower range of about 775 degrees F. to 800 degrees F. For ease inunderstanding and as used hereinafter, the sub-transformationtemperature of the core material refers to a temperature at whichsubstantially no recrystallization of the microstructure will occur.This sub-transformation temperature is also called a stress relievingtemperature. Subsequently, the 84% cold deformed composite isadditionally cold deformed to as much as 97%, wherein the core materialhas a tensile strength in excess of 500,000 p.s.i. If desired, thesheath can be removed from the core such as by chemical dissolution andother methods well known in the art. When the sheath is a nickel-copperalloy, chemical dissolution in nitric acid is quite satisfactory. If thesub-transformation heat treatment is omitted, then the material at a 97%cold deformed state would exhibit a tensile strength in excess of400,000 p.s.i.

In` another preferred embodiment of the invention, the 84% cold deformedcomposite is further cold deformed to approximately 93% to 94%. Thecomposite is heat treated at the sub-transformation temperature range of700 degrees to 825 degrees F. for an approximate period of time such asfour hours. The composite is subsequently cold deformed to 98% and againheat treated at the subtransformation temperature range of 700 degreesF. to 825 degrees F. for an approximate period of time, such as fourhours. The sheath can be removed as described above, with the corehaving a resulting tensile strength ranging from approximately 500,000p.s.i. to approximately 580,000 p.s.i.

In still another embodiment of the invention, the 84% cold deformedcomposite was further drawn to a 97.6% cold deformation state. Thecomposite is heat-treated at the sub-transformation temperature of about700 degrees F. to 825 degrees F. for approximately four hours. Thecomposite is then additionally cold deformed to a 98.7% state. The corematerial exhibited a tensile strength of about 575,000 p.s.i. to 600,000p.s.i. This composite is then heat-treated a second time at asub-transformation temperature, the same as above, for approximatelyabout 4'1/2 hours. The core material then exhibited a tensile strengthin excess of 600,000 p.s.i.

In another embodiment of the invention, the 84% cold deformed compositeis further cold deformed to approximately 97% or more. The composite isthen heat treated at the sub-transformation temperature ranging from 700degrees F. to 825 degrees F. After the sheath is removed similar to themanner described above, the core has a resultant tensile strengthvarying from 475,000 p.s.i. to 525,000 p.s.i.

-In another embodiment of the invention it is possible to form aplurality of ultrahigh strength metal filaments by substituting thecomposite as taught herein for the wire-sheath composite structure ofU.S. Pat. No. 3,277,564, U.S. Pat. No. 3,394,213, and/or U.S.application Ser. No. 6,709, all owned by the assignee hereof. Theteachings of both of these patents are fully incorporated by referenceherein, and are adaptable for use in forming a plurality of ultrahighstrength stainless steel filaments in accordance with the combinedteachings thereof. Depending on the linal application, it is notnecessary to remove the matrix, thereby the end product being acomposite of ultrahigh strength filaments of any desired configurationsurrounded by a metal matrix.

The following examples of specific ultrahigh strength steels made inaccordance with this invention should not be construed in any way tolimit the scope contemplated by this invention.

EXAMPLE I A type of 302 stainless steel rod having a 0.080 inch diameterand an approximate chemical analysis by weight of:

and was surrounded by a Monel K sheath having a 0.115 inch outsidediameter, 0.100 inch inside diameter, and a chemical analysis of nickel,66%; copper, 29%; and aluminum, 3 The rod-sheath composite was drawnthrough a 0.091 inch diameter wire drawing die. The composite wassolution annealed at 1950 degrees F. at a rate of two seconds per mil ofdiameter of the composite and rapidly quenched. The composite was colddrawn through a series of dies with intermediate anneals to a 97.6% coldworked state. The composite was then heat treated at asub-transformation temperature of about 795 degrees F. for approximatelyfour hours. The sheathing material was removed and the resultingstainless wire exhibited a tensile strength of 540,100 p.s.i.

EXAMPLE II Same as Example I except that after the sub-transformationheat treatment of the composite, the composite was further cold drawnfrom the 97.6% state to a 98.7% state of cold work. The sheathingmaterial Was then stripped from the composite and the resultantstainless steel rod exhibited a tensile strength of 592,000 p.s.i. Priorto removing the sheathing the tensile strength was 472,- 800 p.s.i. withthe Monel acting as corrosion resistance coating.

EXAMPLE III Same as Example II except that a final heat treatment at asub-transformation temperature of 795 degrees F. for approximately 41/2hours was employed. The sheathing material was then removed from thecomposite, and the stainless steel wire exhibited a tensile strength of608,000 pounds per square inch.

EXAMPLE IV A type 302 stainless steel rod having a 0.080 diameter and anapproximate chemical analysis by weight of and was surrounded by a MonelK sheath having a 0.115 inch outside diameter, 0.100 inch insidediameter, and a chemical analysis of nickel, 66%; copper, 29%; andaluminum, 3%. The rod-sheath composite was drawn through a 0.091 inchdiameter wire drawing die. The composite was solution annealed at 1950degrees F. at a rate of two seconds per mil of diameter of the compositeand rapidly quenched. The composite was cold drawn through a series ofdies with intermediate anneals prior to achieving a 99.4% cold workedstate. The composite was then stripped of its sheathing material andexhibited a tensile strength of 535,500 p.s.i.

EXAMPLE V The same as Example IV except that after the subtransformationheat treatment of the composite, the composite was further cold Workedfrom a 99.4% cold work state to a 99.6% cold work state. The sheathingmaterial was removed therefrom, with the final stainless steel wireexhibiting a tensile strength of 619,000 pounds per square inch.

EXAMPLE VI Ninety-one (91) type 302 stainless steel rods, each having adiameter of 0.080 inch were placed in Monel 400 tubes, each having a0.115 inch outside diameter and a .100 inch inside diameter. Thechemical composition of the 302 stainless steel by weight was:

Percent Carbon .10 Silicon .46 Manganese .5 Chromium 18.9 Nickel 8.9Phosphorus .018 Sulfur .008 Iron Balance The rod-tube combinations werepacked in a mild steel billet, heated, evacuated to about l"5 torr andsealed. The billet was heated and extruded at 1800 degrees F. with a 16times reduction forming a composite. The composite was then reduced bycold reduction with intermediate anneals. The composite was fullyannealed at a rate of two seconds per mil diameter of the composite. Thecomposite was then cold drawn to a linal diameter of 16.8 mils with eachof the individual rods (now filaments) having an effective cross sectiondiameter of about 1.13 mils. The filaments were at a 93.8% cold workedstate. The strength of the 302 stainless steel laments was found to be393,200 p.s.i. The composite was then heat treated at asub-transformation temperature of about 700 degrees F. for about 16hours. The final strength of the 302 stainless steel filaments was thenfound to be 427,500 p.s.i.

EXAMPLE VII Same as Example VI except that the composite was d colddrawn to a 98.5% cold worked state. The iinal composite diameter was16.8 mils with each of the individual filaments having an effectivecross section dimension of about 1.13 mils. The strength of the 302stainless filaments was found to be 453,700 p.s.i. The composite wasthen heat treated at a sub-transformation temperature of about 700degrees F. for approximately 16 hours. The final strength of the lamentswas found to be 512,200 p.s.i.

It has been found that during the different variations in processingthat the austenite that originally existed in the core materialtransformed intoy at least 50% martensite by a diffusionless phasetransformation. The sheathing material is preselected to have a colddeformation rate that is compatible with the cold deformation rate ofthe core material. It is believed that the additional treatment at thesub-transformation temperature range further inhibits dynamic recoveryand obtains an aging response thereby enhancing the strength alreadyachieved from the deformation hardening. After the sheathing is removed,the ultra-high strength core material may be subjected to a final sizingoperation to obtain a uniform cross sectional geometry.

The general processing steps or operations are graphically depicted inFIG. 8. The ordinate denotes the heat treatment range in temperature andthe abscissa depicts the cycle of operations or steps. Operations A andC indicate cold deforming operations with B indicating an intermediateanneal. Operations indicated as A, B and C are size reducing operationsand can be repeated any number of times. D indicates the solution annealof the core material. E indicates the amount of cold work reductionmeasured in percentage. F indicates the flrst sub-transformation annealwith G indicating subsequent cold reduction. Lastly, H indicates theiinal sub-transformation anneal.

It can readily be seen that the ultrahigh strength materials can becoiled for use in springs, either as tension or compression springs. Inaddition, spiral type watch springs, leaf springs, etc., can also bemade from this material exhibiting such high tensile strength.

Although specific embodiments of the invention have been described, manymodifications and changes may be made, especially in configurations,without departing from the spirit and scope of the invention as definedin the appended claims.

We claim:

1. A high strength stainless steel material having a composition byweight consisting essentially of .15% maximum carbon, 1.5% maximumsilicon, 2% maximum manganese, about 17% to about 19% chromium, about 7%to about 10% nickel, minor amounts of other metals, and the balanceconstituent iron, and characterized in that said material has a tensilestrength in excess of 400,000 p.s.i.

2. The material of claim 1 wherein said material exhibits approximatelynot more than a 20% decrease in strength up to approximately 500 degreesF.

3. The material of claim 1 wherein the material has a perselectedlysized configuration.

4. The material of claim 1 which is formed into a spring.

5. The material of claim 1 further including a tightly adhering sheathmaterial.

6. The material of claim 5 which is formed into a spring.

7. A method of obtaining a tensile strength of at least 400,000 p.s.i.from an 18-8 stainless steel material closely controlling the stepscomprising:

(l) rapidly quenching a solution annealed 18-8 stainless steel materialto prevent carbide precipitation;

(2) cold deforming said material to at least a 75% cold worked state;and

(3) heat treating said cold deformed material at a sub-transformationtemperature to inhibit dynamic recovery and provide an increase instrength.

8. The method of claim 7 further including the step of cold deformingsaid material subsequent to said heat treating.

9. The method of claim 7 wherein the sub-transformation heat treatmenttemperature ranges from 775 F. to 800 F.

10. The method of claim 7 wherein the material is further worked to an84% cold worked state during cold deforming.

11. The method of claim 7 wherein the material is further worked to acold work state during cold deforming.

12. The method of claim 7 wherein the material is further cold worked toat least a 97.6% cold work state during cold deforming.

13. The method of claim 12. wherein after the subtransformationtemperature heat treatment the material is further cold worked toatleast a 98.7% cold worked state with the material exhibiting a tensilestrength in excess of 575,000v p.s.i.

14. A method of obtaining a tensile strength of at least 400,000 p.s.i.from an 18-8 stainless steel material comprising the steps of:

(1) rapidly quenching a solution annealed 18-8 stainless steel materialto prevent carbide precipitation; and

(2) cold deforming said material to at least a 97% cold worked state.

References Cited UNITED STATES PATENTS 2,553,706 5/1951 Goller 14S-12.32,553,707 5/1951 Goller 14S-12.3 3,152,934 10/1964 Lula et al. 14S-12.33,250,611 5/1966 Lula et al. 14S- 12.3 3,359,094 12/1967 Bieber et al.148-37 2,795,519 6/1957 Angel et al. 148-12 3,395,528 `8/1968 Lucht57-145 WAYLAND W. STALLARD, Primary Examiner

